This invention relates to signal processing techniques and, more particularly, to techniques for processing signals which are developed in ultrasonic nondestructive testing systems.
In recent years, nondestructive evaluation techniques have become increasingly important as a means for ascertaining the structural integrity of many different parts and assemblies. One particular branch within the field of nondestructive evaluation involves the use of ultrasonic waves. In the ultrasonic evaluation technique, ultrasonic energy is generated in an object to be tested and the ultrasonic waves which propagate in the object are analyzed, changes in the waves being attributable to the presence and character of flaws or other details of the structure in the object.
Although the use of these ultrasonic techniques has shown great promise, the technique has up to now been somewhat limited in application and has not heretofore been adaptable, for example, to analyze especially difficult evaluation tasks. These shortcomings may be exemplified by way of one particular evaluation problem involving the structural analysis of the wing of the United States Air Force C5A cargo aircraft. This wing incorporates a lapped joint which is connected by a row of fasteners secured through holes provided in the upper and lower halves of the joint. In order to ensure that a particular aircraft is in flyable condition, it is necessary to periodically inspect the area of the joint around these fastener holes to determine whether any cracks of a critical size have developed. Fatigue cracks which form at the fastener holes in the upper half of the joint may be detected by conventional nondestructive evaluation techniques, but the prior art has provided no nondestructive evaluation method, ultrasonic or otherwise, capable of determining whether a repairable fatigue crack has begun to grow from a fastener hole in the lower, inaccessible half of the joint.
Difficulties in evaluating the lower half of the C5A joint have arisen because of the complex geometry of the associated wing structure, which requires the acoustic wave to travel around corners to reach the region of interest, and because the joint includes a layer of sealant whose acoustic properties are substantially different from those of the air frame structure. As a result of these limitations, it has not been possible to effectively inspect the joint without disassembling the wing of the aircraft.
Consequently, as the above example indicates, a need has developed in the art for an ultrasonic testing technique capable of evaluating the structural integrity of objects shaped into complex geometries.